The partitioning of molybdenum and tungsten between ilmenite (ilm) and silicic melts has been determined at 800 degrees C, 1 kbar of pressure, and at two oxygen fugacities. At the oxygen fugacity of the graphite-methane(-water) buffer, D (super ilm/melt) Mo = 0.42, and D (super ilm/melt) W = 0.39. Near the nickel-nickel oxide buffer, D (super ilm/melt) Mo = 0.22 and D (super ilm/melt) W = 0.5. The variations in the partition coefficients with f (sub o 2 ) are significant at the 0.06 and 0.33 levels for molybdenum and tungsten, respectively. The observed variation with oxygen fugacity is tentatively extrapolated to more complex systems and implies that the oxygen fugacity which prevails during magmatic crystallization may play a role in determining the Mo/W ratio in porphyry, skarn, or other hydrothermal mineral deposits. Examination of the literature on molybdenum- and tungsten-bearing mineral deposits indicates that systems with high W/Mo ratios are rather reduced and exhibit high CH 4 /CO 2 ratios in fluid inclusions, low Fe (super +3) /Fe (super +2) ratios in iron-bearing minerals, and low oxygen fugacities in associated igneous rocks. On the other hand, deposits with high Mo/W ratios are associated with rather oxidized magmatic systems. We suggest that f (sub o 2 ) -dependent sequestration of molybdenum and tungsten in ferromagnesian phases during plutonic consolidation exerts a control on the relative efficiencies of removal of these metals from magmas into ore-forming fluids.

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